At present, the majority of the conventional hardness testers adopt the principle of applying stress statically and then measuring the indentation, wherein the Rockwell hardness tester measures the depth of the indentation and directly displays the hardness value; the Brinell hardness tester measures the diameter of the indentation, checks the table or calculates the hardness value; and the Vickers hardness tester measures the diagonal length of the indentation, checks the table or calculates the hardness value. Those meters are desktops and can only be used in labs to test middle and small parts. For a great number of parts produced on a production site, samples must be made or drawn and then sent to the labs to test the hardness. Therefore, those hardness testers have low working efficiency and cannot monitor the hardness of parts in real time on the production site. The Leeb hardness tester, which is quick and convenient but has low accuracy and poor reliability, is adopted for cases where sampling is not allowed and the hardness of the main body of the workpiece must be tested, in particular the weld joint hardness of key members such as the boilers, pressure containers, and pressure pipes. The test results obtained by the Leeb hardness tester shall be converted into a Rockwell, Brinell, or Vickers hardness value. The Leeb hardness tester adopts the rebound test method of dynamic application which is different from that of the Rockwell, Brinell, and Vickers hardness test, so the converted hardness value has a relatively large error.
Plants have an urgent need for a portable digital display hardness tester adopting the Rockwell, Brinell, and Vickers test principle. This hardness tester shall feature portability, simple operation, convenient reading, high efficiency, and high accuracy, and be preferably smart, capable of automatically calibrating the zero point, and automatically correcting the measured value.
An American patent, U.S. Pat. No. 2,544,205, disclosed on Mar. 6, 1951, describes a portable Rockwell hardness tester consisting of a hand wheel, a micrometric nut, a micrometric screw, a drum wheel, a U-shaped elastomer, a hardness indicating gauge, an indenter and a handle. The hand wheel applies test force; the micrometric nut and micrometric screw together with the drum wheel measure the depth of the indentation; the U-shaped elastomer and the hardness indicator gauge together realize the measurement of the test force; and the hardness value is read on the drum wheel.
An America patent, U.S. Pat. No. 2,466,567, disclosed on Apr. 5, 1949, an American patent, U.S. Pat. No. 3,389,597, disclosed on Jun. 25, 1968, and an American patent, U.S. Pat. No. 2,448,645, disclosed on Sep. 7, 1948, respectively describe a portable Rockwell hardness tester adopting similar principles.
The above mentioned patents are mainly applied to measurement of middle and small sized parts. The chain type hardness tester, U.S. Pat. No. 3,389,597, as shown in FIG. 4 can measure large cylindrical parts such as steel pipes and shafts, but the operation is very inconvenient and is usually executed by two persons in cooperation, one holding the tester and one mounting the chain. The hardness scale on the drum wheel is read through a transparent magnifying lens with a reading line. Each scale corresponding to the Rockwell hardness unit represents a 2 μm indentation depth delta value. Due to the distance between the amplifying lens and the drum wheel, the operator may obtain different hardness readings at different view viewing angles. The mentioned testers all have problems such as inconvenient operation, low efficiency, reading errors, failure or inconvenience to test large workpieces.
A Chinese utility model patent, publication number CN201322709Y and application number 200820231921.9, disclosed a portable hardness tester on Oct. 7, 2009. According to this utility model patent, the hardness tester is fixed on the surface of a test sample through two magnetic chucks. The hardness tester has an indentation depth measuring unit consisting of a micrometric nut, a micrometric screw and a reading drum wheel. A U-shaped elastic frame and an indicating gauge work together to indicate the testing force, and then the hardness value is read from the reading drum wheel. In this utility model patent, the hardness indication system is provided with a structure integrating a fixed scale indicator drum and a rotary reading scale drum wheel. Compared with the mentioned American patents, such drum wheel reading structure similar to the micrometer improves the indentation depth measuring accuracy and the hardness value reading accuracy and reduces artificial reading errors. However, this utility model patent still has the following disadvantages:
a. Complicated operation, and low efficiency: the complete test steps include: absorbing the workpiece, setting zero for the scales of the indicator gauge, applying the initial test force, setting zero for the reading drum wheel, applying the total test force, keeping the test force, reducing the test force to the initial test force, reading the hardness value on the drum wheel, and unloading all testing force;
b. Inconvenient reading, low reading accuracy, and operator's reading errors: it is required to carefully operate the drum wheel and read the hardness value represented by the scale on the drum wheel;
c. Low resolution of the displacement measurement and force measurement, low accuracy, and operator's reading errors;
d. Failure to calibrate the hardness value: when the instrument is used, the hardness readings have errors once the U-shaped elastomer undergoes slight deformation or the indicator gauge is impacted, thereby causing measuring errors; in such circumstances, the indicator gauge must be returned to the manufacturer for calibration, and the user cannot calibrate the instrument; and,
e. Low efficiency of the Brinell hardness test, and operator's reading errors: The instrument can only make an indentation on the test sample, and then the user is required to read the indentation diameter with a reading microscope and check the table to obtain the Brinell hardness value. It costs several minutes to complete one test.
A Chinese utility model patent, publication number CN1556387A, application number 200410012621.8, disclosed a portable digital display magnetic hardness tester on Dec. 22, 2004. This utility model patent adopts a case structure containing all main parts. A worm-and-gear structure is adopted to apply the testing force; a main shaft for applying the force is equipped with a displacement sensor for measuring the indentation depth; three force sensors are adopted, and the sum of outputs of those three force sensors represents the testing force; a computer calculates and displays the hardness value after receiving signals from the sensors through electronic circuits. This utility model patent also has the following disadvantages:
a. Complicated structure, big weight, and non-portability.
b. Inconvenient operation, and impractical. Regarding the instrument involved in the utility model patent, the locking hand wheels on both sides are required to be loosened to drop the case rack and then locked each time before testing, operated by two hands to apply the force during the test, and loosened again to support the rack and then fix the rack at the completion of the test. Such complicated working mode is very unacceptable for the operators.
c. Three force sensors are adopted, resulting in high cost.
d. A specific technical solution for the Brinell hardness and Vickers hardness testing is required. Actually, testing of the Brinell hardness and Vickers hardness has higher requirements on the instrument in the aspect of force measuring accuracy, indentation depth measuring accuracy, resolution, and repeatability. Usually, common displacement sensors have difficulties to meet the corresponding requirements.
China Standard GB/T24523-2009 stipulates an advanced Brinell hardness test method: quick indentation (Brinell) hardness test method for metal material. This is the depth measurement of Brinell hardness test, also applicable to the Vickers hardness testing. The principle is as follows:
Apply a certain testing force (including a initial test force and a working test force) to a cemented carbide ball with a certain diameter to press the ball into the surface of the test sample, keep a regulated time, release the working test force, and measure the difference value (depth value) of positions of the indenter before applying and after releasing the working test force on condition of the primary testing force. Measure the corresponding depth values of a plurality of standard Brinell hard blocks, and obtain a relation curve of the indentation depth and the Brinell hardness on certain testing conditions according to the Brinell hardness value and the depth value. When measuring the hardness, check the depth value measured by the hardness tester against the relation curve of the indentation depth and the Brinell hardness to obtain the Brinell hardness value of this material.
Compared with the traditional Brinell hardness test method, this method is an important technical progress. It can realize the quick testing and direct reading of the Brinell hardness, has no need for optical microscopes with a use history of more than 100 years, avoids artificial reading errors, solves the problems in the onsite quick Brinell hardness test of batches of products during production, and even realizes online automatic hardness testing of batches of products in workshops.
The depth-measuring Brinell hardness and Vickers hardness test is a new technique only adopted by some overseas desktop machines. It has not been found in the portable hardness testers yet. The main technical difficulties lie in the simple, light, and small structure of the portable hardness tester, limited inner space, and installation of the common high-accuracy displacement sensor. In addition, it is not economical to install the high-accuracy displacement sensor on a portable hardness tester.
According to the regulations of the related standard, the Brinell hardness measuring scope is 8˜650 HBW; the common scale for the hardness Brinell hardness tester is a 2.5 mm ball and 187.5 kg force; and for a test sample with a hardness value of 650 HBW, the minimum indentation depth is only 0.0365 mm.
The highlight of the portable Vickers hardness tester is the accurate measurement of the hardness of the nitriding layers on large work pieces such as dies and shafts. In the prior art, accurate testing of the hardness of the nitriding layers on large work pieces has been a challenge because the nitriding layers are thin but hard, with a thickness of 0.1˜0.4 mm and a hardness of over 1,000 HV5, and cannot be applied with a large test force. For the 1,000 HV5 nitriding layer, the 5 kg test force is employed, and the indentation depth is only 0.0143 mm. The depth-measurement targets at the depth difference of the indentation, so this numerical value is small.
Realizing high-accuracy and high-resolution displacement measurement on the portable hardness tester is a key for making Brinell and Vickers hardness tests. The inductive displacement transducer and grating displacement transducer usually adopted in the hardness tester field have difficulty meeting the mentioned requirements. Therefore, depth-measuring Brinell and Vickers hardness testing methods have not been applied to portable hardness testers yet.
If such method is realized on a portable hardness tester, the accurate testing of the main body hardness of many key, large, industrial parts becomes possible, making progress in the quality management of the related products.